The Secret Santa Exchange
A group of ten friends decide to exchange gifts as secret Santas. Each person writes his or her name on a piece of paper and puts it in a hat. Then each person randomly draws a name from the hat to determine who has him as his or her secret Santa. The secret Santa then makes a gift for the person whose name he drew.
When it's time to exchange presents, each person walks over to the person he made the gift for and holds his or her left hand in his right hand.
What is the probability that the 10 friends holding hands form a single continuous circle?
When it's time to exchange presents, each person walks over to the person he made the gift for and holds his or her left hand in his right hand.
What is the probability that the 10 friends holding hands form a single continuous circle?
Hint: It's not as difficult as it seems.
It's the number of ways the friends can form a circle divided by the number of ways the names can be drawn out of the hat.
1/10
For a group of n friends, there are n! (n factorial) ways to draw the names out of the hat. Since a circle does not have a beginning and end, choose one person as the beginning and end of the circle. There are now (n-1)! ways to distribute the remaining people around the circle. Thus the probability of forming a single circle is
(n-1)! / n!
Since n! = (n-1)! * n (for n > 1), this can be rewritten as
(n-1)! / (n*(n-1)!)
Factoring out the (n-1)! from the numerator and denominator leaves
1/n
as the probability. Did you answer this riddle correctly?
YES NO
For a group of n friends, there are n! (n factorial) ways to draw the names out of the hat. Since a circle does not have a beginning and end, choose one person as the beginning and end of the circle. There are now (n-1)! ways to distribute the remaining people around the circle. Thus the probability of forming a single circle is
(n-1)! / n!
Since n! = (n-1)! * n (for n > 1), this can be rewritten as
(n-1)! / (n*(n-1)!)
Factoring out the (n-1)! from the numerator and denominator leaves
1/n
as the probability. Did you answer this riddle correctly?
YES NO
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There are actually several distinct solutions. All of them can be constructed as follows:
Draw a nice long straight line.
Draw a second straight line that intersects the first.
Draw three more straight lines making sure each line intersects all the lines youve already drawn and avoiding any of the previous points of intersection. That is, no three lines should intersect at the same point.
With the first four lines, theres only one topologically distinct configuration, but by varying the position of the fifth line, several different distinct configurations can be created. Did you answer this riddle correctly?
YES NO
There are actually several distinct solutions. All of them can be constructed as follows:
Draw a nice long straight line.
Draw a second straight line that intersects the first.
Draw three more straight lines making sure each line intersects all the lines youve already drawn and avoiding any of the previous points of intersection. That is, no three lines should intersect at the same point.
With the first four lines, theres only one topologically distinct configuration, but by varying the position of the fifth line, several different distinct configurations can be created. Did you answer this riddle correctly?
YES NO
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